Arthropod-born viruses constitute important causative agents of emerging hemorrhagic fever diseases. In the past period of support from the Northeast Biodefense Center (NBC) we have demonstrated the presence of viral proteins that antagonize type I IFN-mediated innate immune responses encoded by dengue virus (DENV), the most significant arbovirus pathogen in humans, and Crimean-Congo hemorrhagic fever virus (CCHFV), a tick-born virus of high lethality in humans, and started the characterization of their mechanism of action in virus-infected cells. While DENV encodes multiple proteins responsible for inhibition of type IIFN signaling, one of the most striking features in DENV-infected cells with respect to this signaling pathway is the disappearance of STAT2, a transcription factor required for IFN signaling. Our preliminary data obtained during the last year of support from NBC indicate that the NS5 protein of dengue DENV, known to be the viral RNA-dependent RNA polymerase of this positive strand RNA virus, mediates STAT2 degradation after undergoing proteolytical maturation from the precursor viral polyprotein. In addition, we have described the presence of a protease motif associated with the amino-terminus of the L protein of CCHFV, also known to be the viral RNA-dependent RNA polymerase of this negative strand RNA virus, that removes ubiquitin and the IFN-induced ubiquitin-like molecule ISG15 from conjugated cellular substrates, inhibiting antiviral innate immune responses. During the next period, we now propose to investigate at the molecular level the mechanisms by how these two different viral proteins achieve their inhibitory effects of the host antiviral response, to evaluate the impact of these inhibitory mechanisms in viral pathogenesis and to establish assays for the screening of small compounds that inhibit DENV and CCHFV antagonism of innate immunity, decreasing viral replication and disease. Part of these studies will be performed in collaboration with other projects of the NBC. Our proposed experiments will increase our understanding of viral innate immunity, help with the possible development of an improved DENV mouse model, and hopefully identify new classes of inhibitors for these two hemorrhagic fever viruses.